Field of the Invention
The present disclosure relates to controlling a motor in a sheet conveyance apparatus and an image forming apparatus.
Description of the Related Art
A control method called vector control has heretofore been known as a method for controlling a motor. Vector control controls a motor by controlling a current value in a rotating coordinate system based on a rotation phase of a rotor of the motor. Specifically, there has been known a method for controlling a motor by phase feedback control in which a current value is controlled in the rotating coordinate system in such a manner that a deviation between a command phase and the actual rotation phase of the rotor decreases. There has also been known a method for controlling a motor by speed feedback control in which a current value is controlled in the rotating coordinate system in such a manner that a deviation between a command speed and the actual rotation speed of the rotor decreases.
In vector control, a driving current supplied to a winding of the motor is expressed by a current component (torque current component) that generates torque for rotating the rotor and a current component (exciting current component) that affects an intensity of a magnetic flux passing through the winding. Torque needed for rotation is efficiently generated by controlling the value of the torque current component according to a change in load torque acting on the rotor. This suppresses an increase in motor noise and an increase in power consumption due to surplus torque. This also prevents the rotor from going out of synchronization with an input signal and the motor from entering an incontrollable state (step-out state) because the load torque acting on the rotor exceeds output torque corresponding to the driving current supplied to the winding of the motor. During vector control, the value of the exciting current component is usually controlled so as to become zero. As a result, an increase in power consumption is suppressed.
Rotation of the rotor generates an induced voltage in the winding of each phase of the motor. If an induce voltage occurs in the windings of the motor, the voltage applicable to the windings of the motor decreases. Specifically, suppose, for example, that a power supply for applying a voltage to the windings of the motor has a voltage of 24 V. In such a case, a voltage obtained by subtracting the induced voltage occurring in the windings from the power supply voltage (24 V) can be applied to the windings. The occurrence of the induced voltage in the windings thus makes the voltage applicable to the windings lower than 24 V. The magnitude of the induced voltage increases when the rotation speed of the rotor increases. The higher the rotation speed of the rotor, the lower the voltage applicable to the windings of the motor. As the voltage applicable to the windings of the motor decreases, torque that can be given to the rotor (hereinafter, referred to as torque T) also decreases.
Japanese Patent Application Laid-Open No. 2007-153273 discusses a configuration for weakening an intensity of a magnetic field passing through the windings of the motor (field weakening) by controlling the value of the exciting current component to a negative value corresponding to the rotation speed of the rotor if the rotation speed is a speed threshold or more. There is a one-to-one correspondence between the rotation speed and the value of the excitation current component. Specifically, a predetermined value of the exciting current component is set with respect to a predetermined rotation speed. Field weakening reduces the magnitude of the induced voltage occurring in the windings. This can suppress a decrease in the voltage applicable to the windings and can prevent the torque T from decreasing. The greater the absolute value of the negative value of the exciting current component, the more the torque T can be prevented from decreasing.
A sheet conveyance apparatus for conveying a sheet, such as a recording medium or a document, includes a plurality of loads like rollers for conveying the sheet. A plurality of loads is sometimes driven by one motor. In such a case, for example, the loads are driven by connecting and separating the motor and the loads by a clutch.
FIG. 1 is a block diagram illustrating a configuration of a motor and conveyance rollers serving as loads. As illustrated in FIG. 1, a conveyance roller 701 is driven by a motor 509. The motor 509 and a conveyance roller 702 are connected and separated by a clutch 700.
The clutch 700 connects and separates the motor 509 and the conveyance roller 702 in a state in which the rotor of the motor 509 is rotating at a predetermined speed (constant speed). In other words, a period in which the rotor of the motor 509 rotates at the predetermined speed includes a period during which the motor 509 and the conveyance roller 702 are not connected and a period during which the motor 509 and the conveyance roller 702 are connected. If the motor 509 is driving the conveyance roller 701 in such a manner that the motor 509 and the conveyance roller 702 are separated, load torque corresponding to the conveyance roller 701 acts on the rotor of the motor 509. If the motor 509 and the conveyance roller 702 are connected while the motor 509 is driving the conveyance roller 701, not only the load torque corresponding to the conveyance roller 701 but also load torque corresponding to the conveyance roller 702 acts on the rotor of the motor 509. The load torque acting on the rotor thus increases when the motor 509 and the conveyance roller 702 are connected by the clutch 700. As the loads connected to the motor 509 increase, the load torque acting on the rotor of the motor 509 in rotating the rotor at a predetermined speed increases. In the period in which the rotor rotates at a predetermined speed, the torque T therefore decreases due to the induced voltage occurring in the windings. In the period in which the rotor rotates at a predetermined speed, the load torque acting on the rotor can thus exceed the torque T due to the connection of the loads to the motor 509. If the load torque exceeds the torque T, the rotor becomes unable to rotate.
In the configuration discussed in Japanese Patent Application Laid-Open No. 2007-153273, the rotation speed and the value of the exciting current component have a one-to-one correspondence. In the period in which the rotor rotates at a predetermined speed, the exciting current component is therefore set to a predetermined value corresponding to the predetermined speed.
As described above, in the period in which the rotor rotates at a predetermined speed, the load torque during the period in which the motor and a load are connected is higher than that during the period in which the motor and the load are not connected. In other words, if the configuration discussed in Japanese Patent Application Laid-Open No. 2007-153273 is applied to the control of the motor that is connected to and separated from a load by the clutch, the value of the exciting current component needs to be set in consideration of the load torque during the period in which the motor and the load are connected, in such a manner that the load torque will not exceed the torque T.
The larger the value of the exciting current component in absolute value, the greater the current supplied to the windings of the motor. If the value of the exciting current component is set in consideration of the load torque during the period in which the motor and the load are connected, an unneeded current can be supplied to the windings during the period in which the motor and the load are not connected. As a result, the power consumption increases.